It's possible that 1 Corinthians 15:24-28 is the reason:
Then comes the end, when he delivers the kingdom to God the Father
after destroying every rule and every authority and power.
For he must reign until he has put all his enemies under his feet.
The last enemy to be destroyed is death.
For “God has put all things in subjection under his feet.” But when
it says, “all things are put in subjection,” it is plain that he is
excepted who put all things in subjection under him.
When all things are subjected to him, then the Son himself will also
be subjected to him who put all things in subjection under him, that
God may be all in all.
So the extra 1000 years is to continue working among His people in order to perfect or train their surrender to Jesus in a manner suitable to Him before He hands it all over to the Father.
In Psalms 2:10-12, God is giving instructions to His kings. This passage takes place when God has "set My King on My holy hill of Zion" (Psalms 2:6), after the return of Christ and when He has set up His rulers, which is during the Millennium:
Now therefore, O kings, be wise; be warned, O rulers of the earth.
Serve the LORD with fear, and rejoice with trembling.
Kiss the Son, lest he be angry, and you perish in the way, for his
wrath is quickly kindled. Blessed are all who take refuge in him.
The thousand-year rule = further training in submission and obedience.